1. Field of the Invention
This invention relates to a control system for internal combustion engines, especially-gas engines which employ a pressurized gaseous fuel, such as LPG (liquefied propane gas) and LBG (liquefied butane gas), as fuel, and more particularly to a control system of this kind, which controls the operation of the engine so as to maintain almost constant speed operation.
2. Prior Art
There are widely known gas engines which are constructed such that a pressurized gaseous fuel such as LPG and LBG from a pressurized gaseous fuel source is supplied via a pressure regulator called "zero governor" to the engine for combustion in the combustion chamber.
In the conventional gas engines, the pressurized gaseous fuel from the pressurized gas source is decompressed to atmospheric pressure by the pressure regulator, and the thus decompressed gaseous fuel is supplied to the engine due to vacuum developed in the engine. More specifically, in the conventional gas engines a throttle valve is provided in the intake pipe as a component of a control system of the engine so that an amount of intake air is controlled thereby, and an amount of gaseous fuel commensurate with the thus controlled amount of intake air is supplied to the engine. However, the intake pipe and the throttle valve have to be arranged under some space limitations imposed by the layout of the engine, resulting in a difficulty in miniaturization of the engine and its control system. In addition, the provision of the throttle valve requires the use of a wire link for controlling the throttle valve, making it difficult to design the engine control system simple in construction.
To overcome these difficulties, there has been proposed a gas engine, for example, by Japanese Laid-Open Patent Publication (Kokai) No. 2-23258, in which attempts are made to design the engine compact in size and simple in construction by omitting the use of a throttle valve in the intake pipe and controlling the engine output only by controlling the flow rate of a gaseous fuel supplied to the engine.
The proposed gas engine has its output controlled through control of the gaseous fuel amount alone without control of the intake air amount, utilizing a characteristic of a pressurized gaseous fuel, such as LPG and LBG, that it has a wider range of concentration at which the fuel is combustible than that of liquid fuel such as gasoline and light oil. For example, in the case of gasoline the leanest possible air-fuel ratio (lean limit) at which the air-fuel mixture is combustible is approximately 1.2 in terms of excess air ratio .lambda., while in the case of LPG it is approximately 1.6 in terms of excess air ratio .lambda.. Therefore, the engine output can be controlled based only on the amount of fuel over a relatively wide range of load on the engine, without controlling the amount of intake air.
Further, in the above proposed gas engine, a pressure regulator and a control valve are serially arranged in the intake system at locations downstream of a pressurized gaseous fuel source. The pressure regulator regulates the pressure of gaseous fuel supplied from the pressurized gaseous fuel source to a constant value, in order to prevent the amount of the gaseous fuel from largely changing due to a change in the pressure of the gaseous fuel. The control valve controls the flow rate of the thus pressure-regulated gaseous fuel by controlling the passage area of the gaseous fuel, to thereby control the supply amount of gaseous fuel. The thus controlled amount of gaseous fuel is mixed with intake air, and the mixture is supplied to the engine.
According to the above proposed gas engine, the engine output can be controlled until the air-fuel ratio reaches its lean limit value by reducing the amount of gaseous fuel to be supplied. If the air-fuel ratio becomes leaner than the lean limit value, however, the engine can undergo irregular combustion, resulting in misfiring etc. As a result, fluctuations in the engine output in a low output region cause increased vibrations and irregular combustion, leading to increased emission of unburnt gas components such as hydrocarbon (HC) and hence degraded exhaust emission characteristics of the engine. More specifically, as shown in FIG. 1, the above proposed gas engine has an engine output characteristic that the engine output sharply drops when the engine enters an irregular combustion region where the excess air ratio .lambda. exceeds, for example, 1.6. In the figure, a point A indicates a lean limit point which is a boundary between the irregular combustion region and a normal combustion region in which the engine operation can be normally controlled based on the gaseous fuel amount.
In other words, the proposed gas engine has the disadvantage that when the air-fuel ratio becomes leaner than the lean limit point A, irregular combustion can frequently occur and the emission amount of unburnt gas components such as HC drastically increases, resulting in heavy degradation of exhaust emission characteristics of the engine.
In the proposed gas engine, even if a catalytic converter is provided in the exhaust system for purifying exhaust gases emitted from the engine, the catalytic converter can easily undergo excessive overheating due to the employed gaseous fuel having high concentration. Further, there is a fear that irregular combustion causes fluctuations in the rotational speed of the engine, which results in generation of vibrations, engine stalling, etc. Therefore, the proposed gas engine has limited applications.
Furthermore, since the pressure regulator is arranged in the intake pipe to regulate the pressure of gaseous fuel, a large space for installing-the pressure regulator which is rather large in size, which makes it difficult to design the engine control system compact in size.
Besides, if the kind of gaseous fuel to be used is changed, e.g. when fuel is changed from LPG to LBG, the heating value of which is different from that of LPG, the maximum passage area of the gaseous fuel has to be changed, necessitating exchange of the control valve per se.